1. Field of the Invention
The invention relates to a light wavelength dispersion measuring apparatus and a light wavelength dispersion measuring method for measuring a wavelength dispersion characteristic of an optical fiber.
2. Description of the Related Art
Hitherto, the wavelength dispersion characteristic of the optical fiber has been found from degradation of propagation time relative to the wavelength of an optical signal, namely, the group delay characteristic; generally, the wavelength dispersion characteristic of the optical fiber is represented by propagation time difference per unit length. In optical fiber communications using the optical fiber as a transmission line, distortion occurs in a signal waveform after transmission because of a relationship between the wavelength spread of an optical signal and the wavelength dispersion characteristic of the optical fiber, and the reception characteristic is degraded; this is a problem.
Further, in an optical amplification relay system, the wavelength dispersion characteristic of each optical fiber forming a part of the system is accumulated, thus the effect of a nonlinear phenomenon of the optical fiber caused by the wavelength dispersion characteristic on the transmission characteristic is extremely large. Therefore, to construct a light communication system, it is indispensable to keep track, of the wavelength dispersion characteristic in detail.
FIG. 5 is a block diagram to show the main configuration of a light wavelength dispersion measuring apparatus 20 in a related art. In FIG. 5, the light wavelength dispersion measuring apparatus 20 comprises an electrical oscillator 21 for supplying a single frequency signal, a tunable wavelength Laser diode source 22 provided by placing a plurality of light sources having different light wavelengths, respectively, or a tunable wavelength Laser diode source 22 of a single light source capable of oscillating a plurality of light wavelength signals, an optical fiber directional coupler 23 for branching output of the tunable wavelength Laser diode source 22, a reference optical fiber 24, a measured optical communication line 25 of an optical amplification relay system, etc., connecting one stage or multiple stages of optical fiber using an optical fiber or an optical amplifier, a photoelectric converter 26 for receiving output of the reference optical fiber 24 and converting the optical signal into an electric signal, a photoelectric converter 27 for receiving output of the measured optical communication line 25 and converting the optical signal into an electric signal, and a phase comparator 28 for making a comparison between phases of the electric signals output by the photoelectric converter 26 and 27.
The electrical oscillator 21 supplies a single frequency signal for strength-modulating the optical signal output of the tunable wavelength Laser diode source 22 to the tunable wavelength Laser diode source 22. The strength-modulated optical signal from the tunable wavelength Laser diode source 22 is branched through the optical fiber directional coupler 23. The optical signal branched to one, which is used as a reference signal in the phase comparison, is applied to reference input of the phase comparator 28 through the reference optical fiber 24 which is short and the photoelectric converter 27. The optical signal branched to the other is input to the phase comparator 28 through the measured optical communication line 25 and the photoelectric converter 26. The phase comparator 28 detects the phase difference between the two optical signals. If the phase comparison is made for each wavelength, the group delay characteristic can be obtained.
That is, relative propagation time xcfx84 (xcex) of the measured optical communication line 25 at light wavelength xcex is found according to the following expression (1) from measured phase differencexcex8 (xcex):
xcfx84(xcex)=xcex8(xcex)/2xcfx80fxe2x80x83xe2x80x83(1)
where f is the oscillation frequency of the electrical oscillator 21.
If the relative propagating time xcfx84 (xcex) is converted into unit distance, kilometers (km) and the wavelength xcex is used to enter the horizontal axis and the relative propagation time xcfx84 (xcex) is used to enter the vertical axis, the group delay characteristic is found and further the xcfx84 (xcex) characteristic is differentiated by the wavelength xcex, whereby the wavelength dispersion characteristic can be obtained.
However, the above described light wavelength dispersion measuring apparatus 20 in the related art is adapted to change the wavelength and measure the wavelength dispersion characteristic, and thus involves a problem of prolonging the measurement time. The length of the optical fiber used with the measured optical communication line 25 changes due to change in ambient temperature during measurement, thus causing a measurement error to occur in the relative propagation time xcfx84 (xcex); this is also a problem.
It is an object of the invention to provide a light wavelength dispersion measuring apparatus and light wavelength dispersion measuring method for making it possible to shorten measurement time of measuring a wavelength dispersion characteristic and to correct a measurement error of relative propagation time xcfx84 (xcex) caused by ambient temperature change.
A light wavelength dispersion measuring apparatus comprising:
a short pulse light generator for generating short pulse light;
a first photoelectric conversion unit for executing photoelectric conversion of measured pulse light input from the short pulse light generator through a device under test (DUT) and for outputting a measured pulse signal;
a second photoelectric conversion unit for executing photoelectric conversion of reference pulse light branched and input from the short pulse light generator and for outputting a reference pulse signal;
a first band-pass filter for allowing only an arbitrary frequency component to pass through from the measured pulse signal output from the first photoelectric conversion unit and for outputting a measured frequency signal;
a second band-pass filter for allowing only the same arbitrary frequency component to pass through from the reference pulse signal output from the second photoelectric conversion unit and for outputting a reference frequency signal;
a phase comparison unit for detecting a phase difference between the measured frequency signal output from the first band-pass filter and the reference frequency signal output from the second band-pass filter and for outputting a phase difference signal; and
a wavelength dispersion calculation unit for measuring a group delay amount based on the phase difference signal output from the phase comparison unit and for calculating a wavelength dispersion value.
According to the first aspect of the invention, the first photoelectric conversion unit executes photoelectric conversion of measured pulse light input from the short pulse light generator for emitting short pulse light through the device under test (DUT) and outputs a measured pulse signal, the second photoelectric conversion unit executes photoelectric conversion of reference pulse light branched and input from the short pulse light generator and outputs a reference pulse signal, the first band-pass filter allows only an arbitrary frequency component to pass through from the measured pulse signal output from the first photoelectric conversion unit and outputs a measured frequency signal, the second band-pass filter allows only the same arbitrary frequency component to pass through from the reference pulse signal output from the second photoelectric conversion unit and outputs a reference frequency signal, and the phase comparison unit detects the phase difference between the measured frequency signal output from the first band-pass filter and the reference frequency signal output from the second band-pass filter and outputs a phase difference signal, and then the wavelength dispersion calculation unit measures a group delay amount based on the phase difference signal output from the phase comparison unit and calculates a wavelength dispersion value.
The invention of fourth aspect comprises:
a first photoelectric conversion step of executing photoelectric conversion of measured pulse light of short pulse light emitted from a short pulse light generator, provided through a device under test (DUT) and outputting a measured pulse signal;
a second photoelectric conversion step of executing photoelectric conversion of reference pulse light branched and input from the short pulse light generator and outputting a reference pulse signal;
a first extraction step of allowing only: an arbitrary frequency component to pass through from the measured pulse signal output from the first photoelectric conversion step and extracting a measured frequency signal;
a second extraction step of allowing only the same arbitrary frequency component to pass through from the reference pulse signal output from the second photoelectric conversion step and extracting a reference frequency signal;
a phase comparison step of detecting phase difference between the measured frequency signal output from the first extraction step and the reference frequency signal output from the second extraction step and outputting a phase difference signal; and
a wavelength dispersion calculation step of measuring a group delay amount based on the phase difference signal output from the phase comparison step and calculating a wavelength dispersion value.
Therefore, the optical pulse signals containing the frequencies of an integral multiple of the fundamental frequency are propagated and wavelength dispersion measurement can be executed with good efficiency.
In this case, as in the invention of second aspect, in the light wavelength dispersion measuring apparatus according to the first aspect of the invention, preferably the first band-pass filter comprises a plurality of band-pass filters (for example, band-pass filters BPa1 to BPa3 in FIG. 2) for allowing only a plurality of different arbitrary frequency components to pass through from the measured pulse signal, the second band-pass filter comprises a plurality of band-pass filters (for example, band-pass filters BPb1 to BPb3 in FIG. 2) for allowing only a plurality of the same different arbitrary frequency components to pass through from the reference pulse signal, the phase comparison unit comprises a plurality of phase comparators (for example, phase comparators PCa to PCc in FIG. 2) each for detecting the phase difference between the measured frequency signal and the reference frequency signal of the same frequency output from each of the plurality of band-pass filters in the first band-pass filter and each of the plurality of band-pass filters in the second band-pass filter and outputting a phase difference detection signal for each frequency, and the wavelength dispersion calculation unit calculates the group delay amount based on a plurality of the phase difference detection signals output from the plurality of phase comparators in the phase comparison unit.
Further, in this case, as in fifth aspect of the invention, in the light wavelength dispersion measuring method according to the fourth aspect of the invention, preferably the first extraction step is to allow only different arbitrary frequency components to pass through from the measured pulse signal for extracting a plurality of measured frequency signals, the second extraction step is to allow only the same different arbitrary frequency components to pass through from the reference pulse signal for extracting a plurality of reference frequency signals, the phase comparison step is to detect the phase difference between the measured frequency signal and the reference frequency signal of the same frequency output from the first extraction step and the second extraction step and to output a phase difference detection signal for each frequency, and the wavelength dispersion calculation step is to calculate the group delay amount based on a plurality of the phase difference detection signals output from the phase comparison step.
Therefore, the band-pass filters are connected in parallel and the phase comparators are connected in parallel for making it possible to conduct phase difference measurement of the nth-order harmonics at the same time, so that the wavelength dispersion values of the nth-order harmonics can be calculated at the same time, the group delay amount can be calculated rapidly and accurately in the light wavelength dispersion measuring apparatus, and the measurement time of measuring the wavelength dispersion characteristic can be shortened.
Further, as in a third aspect of the invention, in the light wavelength dispersion measuring apparatus according to the second aspect of the invention, more preferably the wavelength dispersion calculation unit determines whether or not temperature correction of each phase detection value is required based on relative change between the phase difference detection signals output from the plurality of phase comparators in the phase comparison unit, and the light wavelength dispersion calculation unit executes temperature correction.
As in a sixth aspect of the invention, in the light wavelength dispersion measuring method according to the fifth aspect of the invention, preferably the wavelength dispersion calculation step is to determine whether or not temperature correction of each phase detection value is required based on relative change between the phase difference detection signals output from the phase comparison step, and execute temperature correction.
Therefore, the reliability of the light wavelength dispersion measuring apparatus can be enhanced.